1. Technical Field
The present disclosure relates to power amplifiers and, more specifically, to providing adaptive envelope tracking bias voltages for biasing radio frequency (“RF”) power amplifiers.
2. Description of the Related Arts
RF power amplifiers, for example in cell phones, are used to transmit information in the form of modulated radio frequency electromagnetic waves. Power amplifiers are used in many applications such as WiFi, GPS, and the transmission of voice and data. Voice and data applications may also employ multiple frequency bands. The transmission distance is a function of the RF output power. The further the transmission distance, the higher the required output power, and the more battery power is consumed.
Power amplifiers (“PA”) consume most of the battery power in many usage cases, for example when a cell phone constantly transmits data to the nearby cell towers. The existing power supply architecture in cell phones uses the system supply voltage (e.g., the battery voltage) as the maximum bias voltage to the power amplifiers. Under this concept, the PA is designed to operate at peak efficiently for maximum powers when biased at the system supply voltage. However, under this design, RF PAs have overall low efficiency in many applications, such as smartphones, tablets, etc. This is because, when the RF PA is biased at the system supply voltage, the system and RF PA are designed for efficiency only when there is an RF signal of maximum power. However, for most of the time, RF PAs do not operate at full power. The average power for an RF PA typically is 1/2 to 1/7 of its saturated power. Accordingly, a large amount of DC power is wasted when the RF PA operates at these lower powers.
To improve the RF PA efficiency at lower power levels, envelope tracking (ET) or average power tracking (APT) techniques are used. Envelope tracking adjusts the bias voltage applied to the PA to increase the PA operating efficiency. In other words, the power supply voltage is adjusted to ensure that the PA is operating at peak efficiency for the power required at each instant of transmission. The envelope is the magnitude of the modulated RF signal. The speed of the envelope variation is typically in the MHz range and increases in wider bandwidth modulation applications. One approach is to use a linear regulator (e.g., LDO) and a buck-boost DC-converter. However, this approach has many disadvantages. The PA's overall efficiency is compromised because of the linear regulator's low efficiency. Moreover, when the bandwidth of LTE or other RF signals increases (e.g., reaching 40 MHz or 60 MHz under carrier aggregation), linear regulators typically will have difficulty to meet the signal envelope speed, and degradations in linearity may become unacceptable.
Furthermore, PAs must meet linearity requirements at high output power while operating at system supply voltage (e.g., 3.4V in cell phones). Cell phones output high power less frequently than low power, and PAs in cellphones often step down the supply voltage in order to bias the PA at a point that increases the efficiency. However, stepping down the supply voltage induces power loss. The lower the output voltage is, the lower the efficiency of the envelope tracking power supply system.
Accordingly, there is a need for PAs to work more efficiently across a range of power conditions.